This invention pertains generally to antennas for radio frequency energy, and more particularly to directional antennas wherein parasitic elements are used to control the direction of a beam from an antenna.
In guided missile (or simply missile) applications, fuse and link antennas often are required to be mounted conformally with the generally cylindrical shape of a missile. Antennas which adapt easily to conformal mounting usually produce beams with main lobes directed normally (or broadside to) the missile, whereas the required direction of main lobes of beams for fuse and link antennas is usually not normal (or broadside) to the missile. Thus, the main lobes of fuse antenna beams are typically pointed forward of the missile, while the main lobes of link antenna beams are usually pointed aft, say in a beam direction approximately twenty degrees off of normal. To accomplish such an end, known link antennas are usually made of components that occupy critical area internally of the missile. The mass and volume of all components within the missile are critical to performance, and any decrease in the size and number of components is highly desirable.
It is known in the art that microstrip patch antennas have a low profile and may be made conformal to a missile. Unfortunately, most patch antennas produce an antenna beam normally disposed to the aperture of the antenna. Different approaches have been used to change the antenna beam direction. Multiple patch antenna arrays have been used to steer the antenna beam direction. Such arrays have been built by using a stripline distribution network; however, such a network is complicated, with many connections required. A less complicated technique is desirable.
It is also known in the art that parasitic elements may be used to control the direction of the beam of an antenna. For example, the well-known "Yagi" antenna uses parasitic elements in combination with at least one active element to control the direction of a beam. A similar technique is known for use with parasitic slot array antennas, as described in an article by R. J. Coe and G. Held, I.E.E.E. Transactions on Antennas and Propagation, Vol. Ap-12, No. 1, pp. 10-16, January 1964. In such an array, a reflector element and a director element are formed by cavity-tuned parasitic slots so that when a driven element (a slot) is excited, a beam is formed in the direction of the director in the plane of the elements. The parasitic slot array provides a flush mounting antenna suitable for an application where no projection above a plane surface is required. However, as noted previously, a fuse or link antenna usually requires an antenna beam direction approximately twenty degrees off of normal or broadside of the face of the antenna so the parasitic slot array is hardly one to be used in a missile.